NCCCO Anti-Two-Block Devices and Safety Systems 3 — Questions and Answers
Question 1: When an anti-two-block (A2B) device activates during a lift, the crane operator should FIRST:
- Lower the load immediately to relieve tension
- Override the A2B device to complete the lift
- Stop all hook and boom motions that would worsen the condition (Correct answer)
- Retract the boom to create more headroom
Correct answer: Stop all hook and boom motions that would worsen the condition
Upon A2B activation, the operator must immediately stop all crane motions that would further reduce the distance between the hook block and boom tip.
Question 2: Which type of A2B sensor is MOST commonly used on modern hydraulic cranes?
- Mercury tilt switch
- Weight-actuated switch with a hanging weight on the load block (Correct answer)
- Laser distance sensor
- Infrared proximity detector
Correct answer: Weight-actuated switch with a hanging weight on the load block
Weight-actuated (pendulum or bob-weight) switches are the most common A2B devices, where the hanging weight lifts when the hook block contacts the boom tip.
Question 3: A2B devices are required to prevent unintended contact between the load block and the:
- Counterweight
- Boom tip or uppermost sheave (Correct answer)
- Outrigger pads
- Hoist drum
Correct answer: Boom tip or uppermost sheave
The A2B device prevents the load block or headache ball from contacting the boom tip or the uppermost load block sheave.
Question 4: When should a crane operator test the A2B device at the start of a shift?
- Only when a new operator takes over
- Only after maintenance has been performed
- Before beginning any lifting operations (Correct answer)
- At the end of each shift
Correct answer: Before beginning any lifting operations
OSHA and ASME B30.5 require operators to verify that all safety devices, including A2B devices, are functional before beginning lifting operations.
Question 5: If an A2B warning alarm sounds but the cut-out function does not engage, the operator should:
- Continue the lift while monitoring the alarm
- Immediately stop operations and tag the crane out of service (Correct answer)
- Manually control hoist speed to compensate
- Reset the A2B device and continue
Correct answer: Immediately stop operations and tag the crane out of service
A non-functional cut-out means the primary two-blocking protection has failed; the crane must be taken out of service until repaired.
Question 6: Which ASME standard specifically covers safety requirements for mobile and locomotive cranes, including A2B requirements?
- ASME B30.2
- ASME B30.5 (Correct answer)
- ASME B30.10
- ASME B30.22
Correct answer: ASME B30.5
ASME B30.5 is the standard for mobile and locomotive cranes and includes requirements for anti-two-block devices.
Question 7: Two-blocking is most likely to occur when the operator is:
- Lowering a load at full boom extension
- Raising the hook rapidly with the boom raised and retracted (Correct answer)
- Slewing with a suspended load
- Picking a load at maximum rated capacity
Correct answer: Raising the hook rapidly with the boom raised and retracted
Raising the hook at high speed with a short, raised boom reduces the distance to the boom tip rapidly, creating high two-block risk.
NCCCO Anti-Two-Block Devices and Safety Systems 3
When an anti-two-block (A2B) device activates during a lift, the crane operator should FIRST: